1. Field of the Invention
The present invention relates to a bearing device of a motor to be incorporated in a device such as a video equipment.
2. Description of the Prior Art
A video tape recorder is generally used as a video equipment, and a motor for rotating video tape at a predetermined speed is incorporated in the video tape recorder. An example of a bearing device for the motor is shown in FIG. 7 as it is disclosed in JP-A-8-284965. Under a holder 31, a collar unit 32 is protrudes. On the outer periphery of the collar unit 32, fixing units 38 and mounting units 39 are arranged alternately with equal spacing. In engaging sectors 34a and 34b provided on the upper portion and lower portion of the holder 31 respectively, bearing bodies 33a and 33b are engaged, and these are unremovably held by presser washers 35a and 35b. In particular, dimension with high precision are required for the engaging sectors 34a and 34b, and the bearing device is usually produced by die-cast molding from various types of alloys such as aluminum, zinc, magnesium, etc.
In the product produced by die-cast molding, solidifying time varies when thickness is not uniform, and it is known that a slight recess (also called "recess mark") is likely to occur on a thicker portion. For this reason, on the engaging unit on the upper portion of the holder, thickness of the peripheral wall is uniform and recess does not occur almost at all, and it is possible to maintain circularity (difference of maximum value and minimum value of inner diameter) within 6 .mu.m. On the other hand, as shown in FIG. 8, on the lower portion of the holder 31, there are provided reinforcing pieces 42 directed toward mounting units 39 and also reinforcing ribs 43 reaching fixing units 38 in radial direction, and thickness of the peripheral wall of the engaging unit 34b on the lower portion becomes uneven. Thus, recesses 49 are likely to occur on the engaging unit 34b as shown in FIG. 9.
When there are recesses on the engaging unit, gap is generated between the engaging unit and the bearing bodies. When oil is supplied to the bearing bodies (oil-impregnated metal), oil tends to enter the recesses, and this reduces the quantity of oil to enter between motor rotation shaft and the engaging unit to smoothen the rotation and also decreases supporting ability of the bearing bodies. In particular, the recesses may cause slightly eccentric rotation of the rotation shaft of the motor, and this may cause errors in video tape feeding speed.
To solve the above problems, it is an object of the present invention to provide a bearing device, by which it is possible to rotate a rotation shaft of a motor smoothly at a constant speed without impairing the problem characteristics even when load is applied on the rotation shaft of the motor when the device is incorporated in a video equipment.
According to claim 1 the present invention, a bearing device is provided, which comprises a holder designed in cylindrical shape, a collar unit extending in outer peripheral direction near a first end in axial direction of the holder, fixing units and mounting units alternately arranged along the circumference of outer periphery of the collar unit, and engaging sectors of bearing bodies on each of the first end and a second end on the other end of a bearing bore of the holder, whereby an annular groove formed in the axial direction from the first end of the holder is provided so that thickness of the peripheral wall forming the engaging unit of the bearing bodies of the holder is even over the entire periphery. Here, the first end of the holder means an end closer to the mounting side of the holder, and the second end means an end opposite to the mounting side.
The invention, the annular groove reaches the designed depth of the engaging unit from the bottom surface of the collar unit. Reaching the designed depth of the engaging sector means that it may enter slightly deeper in the axial direction than the depth surface of the engaging unit or it may agree with the depth surface or may be slightly shallower.
In the device according to the invention, a reinforcing unit to support the peripheral wall of the engaging unit may be provided in the annular groove. Axial length of the reinforcing unit may agree with groove depth of the annular groove or may be slightly shallower or may reach the first end of the-holder.
In the device according to the present invention, the difference between maximum value and minimum value of inner diameter of the peripheral wall of the engaging unit on the first end of the holder is not more than 6 .mu.m.